Review of the Prior Art
Several different forms of devices are known for measuring the location of the level of a liquid in a tank or other vessel. They are used as control or data gathering mechanisms in diverse industrial, laboratory and other applications. They include various kinds of float operated mechanisms, capacitative mechanisms, resistance mechanisms, and acoustic mechanisms, all of which require the presence in the tank of at least part of the structure of the mechanism.
There are many industrial processes in which the level of a liquid in a tank or vessel must be measured but in which it is undesirable for the level measurement equipment to be in the tank itself. The manufacture of pharmaceuticals and food products are examples of such processes; concerns about purity of process liquids outweigh the convenience of level measurement devices in tanks for such instances. In other instances, the process liquid may be corrosive and hostile to level sensing or measuring devices. As a result, costly and often unreliable systems for measuring liquid levels from the exterior of a tank must be used, or a series of devices must be used at different vertical stations along the tank exterior. Gamma ray devices, for example, are costly and are subject to frequent inspections by regulatory agencies; they can not be used in food manufacturing processes. Doppler effect nonintrusive level sensors must be used in groups along the exterior of a tank, because each device provides information that a liquid is or is not present inside the tank at the level where the device is mounted to the outside of the tank.
Intrusive, i.e., inside the tank, acoustic liquid level sensing devices are well known, reliable and effective in many applications. Acoustic level sensors use an acoustic energy transceiver inside a tank to transmit a pulsed sound signal vertically toward a liquid surface and to generate an echo signal upon receipt of an echo or reflection of the transmitted sound pulse from the liquid surface. The elapsed time between transmission of the sound pulse and receipt of the echo is measured and halved; that information, coupled with knowledge of the velocity of sound between the transceiver and the liquid surface, provides a measurement of the distance from the transceiver to the liquid surface. The transceiver can be located in the liquid below the surface to direct the transmitted sound pulse upwardly, or the transceiver can be located above the liquid surface to radiate sound pulses downwardly through air or some other fluid above the liquid surface.
The velocity of sound in water at different temperatures is known. The velocity of sound in a few other liquids is also known. For most liquids, however, knowledge of the velocity of sound through the liquid is either limited or non-existent. For that reason, target-type intrusive acoustic level sensors have been developed. They include a target member spaced a known short distance from the sound pulse source toward the liquid surface. As a sound pulse is generated, a portion of the radiated sound pulse is reflected by the target back to the source which then operates as an echo receiver; the remainder of the radiated pulse passes on to the liquid surface where it is reflected back to the source operating as a receiver. The time between pulse generation and target echo receipt is measured for a known distance, thereby providing information about the velocity of sound in the intervening medium, and that velocity figure is applied to the echo from the liquid surface to describe the distance from the sound source to the liquid surface. However, a target-type level sensor cannot be used from outside a liquid storage tank because the target must be inside the tank to provide useful information about the velocity of sound in the medium between the sound source and the liquid surface.
A nonintrusive acoustic liquid level sensor for use on the bottom of a tank has recently been commercially introduced by Magnetrol International. It is understood that that sensor has an operating frequency which is preset by the manufacturer for a specified liquid of interest, and the signal processing circuitry associated with that sensor is defined by the manufacturer for the velocity of sound in that liquid of interest as specified by the customer or user. The customer must also specify to the manufacturer the thickness of the tank wall to which the sensor will be mounted and the material of the tank wall. That sensor, therefore, is effectively custom made and is usable by a customer only with the specified tank and liquid unless the sensor is returned to the manufacturer for resetting to different use conditions. That sensor cannot be stocked by a distributor for sale to any of several customers, nor can a user efficiently change the location of use or liquid of interest as may be desired.
It is apparent that a need exits for a versatile, reliable, effective and relatively uncomplicated non-intrusive liquid level sensor which can be used with tanks having a wide range of wall thicknesses and materials and which is adjustable, if needed, by a user to suit various tank and liquid usage situations. Such a liquid level sensor can be used with virtually any liquid of interest and a wide range of tanks. It can be manufactured as a standard product, rather than as a custom product, with significant reductions in manufacturing cost. Such a sensor can be inventoried and marketed more effectively, thereby further reducing costs to users and better serving the needs of users. Such a sensor, because prior knowledge of sound velocity in a liquid of interest is not required, can be used on test or on-approval bases by prospective users to ascertain sensor suitability for specific needs, and can also be used with tanks which contain different liquids at different times.